32 research outputs found
ANS hard X-ray experiment development program
The hard X-ray (HXX) experiment is one of three experiments included in the Dutch Astronomical Netherlands Satellite, which was launched into orbit on 30 August 1974. The overall objective of the HXX experiment is the detailed study of the emission from known X-ray sources over the energy range 1.5-30keV. The instrument is capable of the following measurements: (1) spectral content over the full energy range with an energy resolution of approximately 20% and time resolution down to 4 seconds; (2) source time variability down to 4 milliseconds; (3) silicon emission lines at 1.86 and 2.00keV; (4) source location to a limit of one arc minute in ecliptic latitude; and (5) spatial structure with angular resolution of the arc minutes. Scientific aspects of experiment, engineering design and implementation of the experiment, and program history are included
Orbital dynamics of Cygnus X-3
Orbital-phased-resolved infrared spectra of Cygnus X-3 in outburst and
quiescence, including tomographic analysis, are presented. We confirm the
phasing of broad HeII and NV lines in quiescence, such that maximum blue shift
corresponds to the X-ray minimum at phase = 0.00 +/- 0.04. In outburst,
double-peaked HeI structures show a similar phasing with two significant
differences: (a) although varying in relative strength, there is continuous
line emission in blue and red peaks around the orbit, and (b) an absorption
component, ~1/4 of an orbit out of phase with the emission features, is
discerned. Doppler tomograms of the double-peaked profiles are consistent with
a disk-wind geometry, rotating at velocities of 1000 km/s. Regrettably, the
tomography algorithm will produce a similar ring structure from alternative
line sources if contaminated by overlying P Cygni profiles. This is certainly
the case in the strong 2.0587 micron HeI line, leading to an ambiguous solution
for the nature of double-peaked emission. The absorption feature, detected 1/4
of an orbit out of phase with the emission features, is consistent with an
origin in the He star wind and yields for the first time a plausible radial
velocity curve for the system. We directly derive the mass function of the
system, 0.027 M_sun. If we assume a neutron star accretor and adopt a high
orbital inclination, i > 60 degrees, we obtain a mass range for the He star of
5 M_sun < M_WR < 11 M_sun. Alternatively if the compact object is a black hole,
we estimate M_BH < 10 M_sun. We discuss the implications of these masses for
the nature and size of the binary system.Comment: Accepted for publication in ApJ main journa
Exploring the spreading layer of GX 9+9 using RXTE and INTEGRAL
We have fitted ~200 RXTE and INTEGRAL spectra of the neutron star LMXB GX 9+9
from 2002-2007 with a model consisting of a disc blackbody and another
blackbody representing the spreading layer (SL), i.e. an extended accretion
zone on the NS surface as opposed to the more traditional disc-like boundary
layer. Contrary to theory, the SL temperature was seen to increase towards low
SL luminosities, while the approximate angular extent had a nearly linear
luminosity dependency. Comptonization was not required to adequately fit these
spectra. Together with the ~70 degree upper bound of inclination implied by the
lack of eclipses, the best-fitting normalization of the accretion disc
blackbody component implies a distance of ~10 kpc, instead of the usually
quoted 5 kpc.Comment: 14 pages, 10 figures, accepted for publication in MNRA
Time-sequenced Multi-Radio-Frequency Observations of Cygnus X-3 in Flare
Multifrequency observations from the VLA, VLBA and OVRO Millimeter Array of a
major radio outburst of Cygnus X-3 in 2001 September are presented, measuring
the evolution of the spectrum of the source over three decades in frequency,
over a period of six days. Following the peak of the flare, as the intensity
declines the high-frequency spectrum at frequency nu steepens from nu^{-0.4} to
nu^{-0.6}, after which the spectral index remains at this latter terminal
value; a trend previously observed but hitherto not satisfactorily explained.
VLBA observations, for the first time, track over several days the expansion of
a sequence of knots whose initial diameters are approximately 8
milliarcseconds. The light-crossing time within these plasmons is of the same
order as the time-scale over which the spectrum is observed to evolve. We
contend that properly accounting for light-travel time effects in and between
plasmons which are initially optically thick, but which after expansion become
optically thin, explains the key features of the spectral evolution, for
example the observed timescale. Using the VLBA images, we have directly
measured for the first time the proper motions of individual knots, analysis of
which shows a two-sided jet whose axis is precessing. The best-fit jet speed is
roughly beta = 0.63 and the precession period is about 5 days, significantly
lower than fitted for a previous flare. Extrapolation of the positions of the
knots measured by the VLBA back to zero-separation shows this to occur
approximately 2.5 days after the detection of the rise in flux density of
Cygnus X-3.Comment: 23 pages, 10 figures, accepted by Ap
The Hardness-Intensity Diagram of Cygnus X-3: Revisiting the Radio/X-Ray States
Cygnus X-3 is one of the brightest X-ray and radio sources in the Galaxy, and
is well known for its erratic behaviour in X-rays as well as in the radio,
occasionally producing major radio flares associated with relativistic
ejections. However, even after many years of observations in various wavelength
bands Cyg X-3 still eludes clear physical understanding. Studying different
emission bands simultaneously in microquasars has proved to be a fruitful
approach towards understanding these systems, especially by shedding light on
the accretion disc/jet connection. We continue this legacy by constructing a
hardness-intensity diagram (HID) from archival Rossi X-ray Timing Explorer data
and linking simultaneous radio observations to it. We find that surprisingly
Cyg X-3 sketches a similar shape in the HID to that seen in other transient
black hole X-ray binaries during outburst but with distinct differences.
Together with the results of this analysis and previous studies of Cyg X-3 we
conclude that the X-ray states can be assigned to six distinct states. This
categorization relies heavily on the simultaneous radio observations and we
identify one new X-ray state, the hypersoft state, similar to the ultrasoft
state, which is associated to the quenched radio state during which there is no
or very faint radio emission. Recent observations of GeV flux observed from Cyg
X-3 (Tavani et al. 2009; Fermi LAT Collaboration et al. 2009) during a soft
X-ray and/or radio quenched state at the onset of a major radio flare hint that
a very energetic process is at work during this time, which is also when the
hypersoft X-ray state is observed. In addition, Cyg X-3 shows flaring with a
wide range of hardness.Comment: 17 pages, 9 figures, accepted for publication in MNRA
The Reoccurrence of mHz QPOs in Cygnus X-3
We have re-analyzed archival RXTE data of the X-ray binary Cygnus X-3 with a
view to investigate the timing properties of the source. As compared to
previous studies, we use an extensive sample of observations that include all
the radio/X-ray spectral states that have been categorized in the source
recently. In this study we identify two additional instances of Quasi-Periodic
Oscillations that have centroid frequencies in the mHz regime. These events are
all associated to a certain extent with major radio flaring, that in turn is
associated with relativistic jet ejection events. We review briefly scenarios
whereby the Quasi-Periodic Oscillations may arise.Comment: 5 pages, 3 figures, accepted for publication in MNRA
Cygnus X-3 and the problem of the missing Wolf-Rayet X-ray binaries
Cygnus X-3 is a strong X-ray source (L_X about 10^38 erg/s) which is thought
to consist of a compact object, accreting matter from a helium star. We find
analytically that the estimated ranges of mass-loss rate and orbital-period
derivative for Cyg X-3 are consistent with two models: i) the system is
detached and the mass loss from the system comes from the stellar wind of a
massive helium star, of which only a fraction that allows for the observed
X-ray luminosity is accreted, or ii) the system is semidetached and a
Roche-lobe-overflowing low- or moderate-mass helium donor transfers mass to the
compact object, followed by ejection of its excess over the Eddington rate from
the system. These analytical results appear to be consistent with evolutionary
calculations. By means of population synthesis we find that currently in the
Galaxy there may exist ~1 X-ray binary with a black hole that accretes from a
>~ 7 MSun Wolf-Rayet star and ~1 X-ray binary in which a neutron star accretes
matter from a Roche-lobe-overflowing helium star with mass <~ 1.5 MSun. Cyg X-3
is probably one of these systems.Comment: 12 pages, 6 figures, accepted by A&
A High Statistics Search for Ultra-High Energy Gamma-Ray Emission from Cygnus X-3 and Hercules X-1
We have carried out a high statistics (2 Billion events) search for
ultra-high energy gamma-ray emission from the X-ray binary sources Cygnus X-3
and Hercules X-1. Using data taken with the CASA-MIA detector over a five year
period (1990-1995), we find no evidence for steady emission from either source
at energies above 115 TeV. The derived upper limits on such emission are more
than two orders of magnitude lower than earlier claimed detections. We also
find no evidence for neutral particle or gamma-ray emission from either source
on time scales of one day and 0.5 hr. For Cygnus X-3, there is no evidence for
emission correlated with the 4.8 hr X-ray periodicity or with the occurrence of
large radio flares. Unless one postulates that these sources were very active
earlier and are now dormant, the limits presented here put into question the
earlier results, and highlight the difficulties that possible future
experiments will have in detecting gamma-ray signals at ultra-high energies.Comment: 26 LaTeX pages, 16 PostScript figures, uses psfig.sty to be published
in Physical Review
A giant radio flare from Cygnus X-3 with associated Gamma-ray emission
With frequent flaring activity of its relativistic jets, Cygnus X-3 is one of
the most active microquasars and is the only Galactic black hole candidate with
confirmed high energy Gamma-ray emission, thanks to detections by Fermi/LAT and
AGILE. In 2011, Cygnus X-3 was observed to transit to a soft X-ray state, which
is known to be associated with high-energy Gamma-ray emission. We present the
results of a multi-wavelength campaign covering a quenched state, when radio
emission from Cygnus X-3 is at its weakest and the X-ray spectrum is very soft.
A giant (~ 20 Jy) optically thin radio flare marks the end of the quenched
state, accompanied by rising non-thermal hard X-rays. Fermi/LAT observations (E
>100 MeV) reveal renewed Gamma-ray activity associated with this giant radio
flare, suggesting a common origin for all non-thermal components. In addition,
current observations unambiguously show that the Gamma-ray emission is not
exclusively related to the rare giant radio flares. A 3-week period of
Gamma-ray emission is also detected when Cygnus X-3 was weakly flaring in
radio, right before transition to the radio quenched state. No Gamma rays are
observed during the ~ one-month long quenched state, when the radio flux is
weakest. Our results suggest transitions into and out of the ultrasoft X-ray
(radio quenched) state trigger Gamma-ray emission, implying a connection to the
accretion process, and also that the Gamma-ray activity is related to the level
of radio flux (and possibly shock formation), strengthening the connection to
the relativistic jets.Comment: Accepted for publication in MNRAS. 10 pages 5 figures, 1 tabl
What ignites on the neutron star of 4U 0614+091?
[abridged] The LMXB 4U 0614+091 is a source of sporadic thermonuclear (type
I) X-ray bursts. We find bursts with a wide variety of characteristics in
serendipitous wide-field X-ray observations by EURECA/WATCH, RXTE/ASM,
BeppoSAX/WFC, HETE-2/FREGATE, INTEGRAL/IBIS/ISGRI, and Swift/BAT, as well as
pointed observations by RXTE/PCA and HEXTE. Most of them reach a peak flux of
~15 Crab, but a few only reach a peak flux below a Crab. One of the bursts
shows a very strong photospheric radius-expansion phase. This allows us to
evaluate the distance to the source: 3.2 kpc. The burst durations are between
10 sec to 5 min. However, after one of the intermediate-duration bursts, a
faint tail is seen to at least ~2.4 hours after the start of the burst. One
very long burst lasted for several hours. This superburst candidate was
followed by a normal type-I burst only 19 days later. This is, to our
knowledge, the shortest burst-quench time among the superbursters. A superburst
in this system is difficult to reconcile if it accretes at ~1% L_Edd. The
intermediate-duration bursts occurred when 4U 0614+091's persistent emission
was lowest and calm, and when bursts were infrequent (on average one every
month to 3 months). The average burst rate increased significantly after this
period. The maximum average burst recurrence rate is once every week to 2
weeks. The burst behaviour may be partly understood if there is at least an
appreciable amount of helium present in the accreted material from the donor
star. If the system is an ultra-compact X-ray binary with a CO white-dwarf
donor, as has been suggested, this is unexpected. If the bursts are powered by
helium, we find that the energy production per accumulated mass is about 2.5
times less than expected for pure helium matter.Comment: 24 pages, 15 figures, accepted for publication in Astronomy and
Astrophysic